1. Field of the Invention
The present invention generally relates to systems and methods for controlling operation of a center pivot irrigation system and, more particularly, to a center pivot irrigation system that is configured to provide constant depth of water application in selected areas of irrigated land (e.g., to avoid application of valuable water on non-productive areas of a field, avoid watering a road, and so on) even with changing water flow and/or pressures being input by the center pivot irrigation system pump(s). The center pivot irrigation system is controlled so as to irrigate the farmer-designated or desired areas of the field, which may cause more or less of the irrigator's nozzles to be used but doing so with a “constant” flowrate so that the number of valves open at any time adds up to approximately the same total flow.
2. Relevant Background
In many areas of the world, the agricultural industry relies upon effective irrigation to raise crops. Irrigation may be used due to limited rainfall or rainfall that is not adequate for a particular crop. Further, irrigation may be utilized due to the variability in the timing of rainfall as many crops require a relatively consistent amount of water over their growing period.
Center pivot irrigation is one of the most water-efficient irrigation techniques. Center pivot irrigation is a form of overhead sprinkler irrigation that makes use of a long sprinkler arm formed of several segments of pipe joined together and supported by trusses. The pipe segments are mounted on wheeled towers each typically driven by an electric motor. Sprinklers or sprinkler heads are spaced apart along the length or span of the sprinkler arm. The sprinkler arm is centrally anchored such that the sprinkler arm moves in a circular pattern, and water is fed into the sprinkler arm at the center of the circle via one, two, or more pumps that may be positioned anywhere between the sprinkler arm and a water source such as an aquifer.
The outside set of wheels on the arm may set the master pace for the rotation such as once every three days. The inner sets of wheels can be mounted at hubs or towers between two pipe segments, and angle sensors can be used to detect when the bend at the joint between the two segments exceeds a certain maximum threshold as an indication of when the electric motor driving the inner wheel sets should be rotated to keep the pipe segments generally aligned along the arm span. Center pivot sprinkler arms are typically less than about 1600 feet (or 500 meters) in length (i.e., irrigated circle radius) with a common size being 440 feet (or a quarter mile or 400 meters) length or circle radius, but many pivot sprinkler arms may be much longer than 500 meters in length.
Most center pivot irrigation systems use sprinklers that hang down from the sprinkler arm pipe segments so that the sprinkler heads are positioned a few feet above the ground or crop so as to limit evaporative losses and wind drift. There are many different sprinkler head or nozzle configurations that may be used including a static plate, a moving plate, and so on with a rotator-style pivot applicator sprinkler head or nozzle being one of the more popular in present irrigation systems. The rotator-style sprinkler head or nozzle is adapted to work properly with input water at a particular pressure (or within a desired range and not very well or at all if too far below this pressure), and a pressure regulator may be provided upstream of each nozzle or sprinkler head to ensure that each is operating at the correct design pressure (not over pressurized).
It is typically an operational goal of a center pivot irrigation system to provide uniform application of water, which may include effluent, chemicals, and fertilizers, and to avoid applying too much water in one application (e.g., by running too slowly) so as to reduce run-off and leaching. To achieve such uniform application, the center pivot irrigation system generally requires an even emitter flow rate across the length or span of the sprinkler arm. Since the outer-most pipe segments and wheeled towers travel farther in a given time period (e.g., at a higher speed) than the inner-most pipe segments and wheeled towers, nozzle sizes may be smallest at the inner spans and increase in size with distance from the pivot point such that a greater flow of water is output at positions on the sprinkler arm that are moving the fastest (and a lower flow of water is output at positions on the sprinkler arm that are moving the slowest).
While center pivot irrigation systems are considered to be highly efficient systems that are useful in conserving water, there remains a number of challenges associated with their use to irrigate crops. One challenge is that the amount of water available to the center pivot irrigation system may vary over the growing season. For example, water may initially be available at a flow rate of 550 to 600 gallons per minute (GPM) but later drop down to 400 to 450 GPM. Presently, one solution to this problem of varying input water flow rates is to change out the nozzle sizes along the entire length of the sprinkler arm to continue to have uniform output flows and to have nozzles that operate well at lower pressures. Keeping a separate set of nozzles and changing them out midseason can be very expensive and time consuming. Another approach to addressing this problem is to utilize a variable speed pump(s), which enable valve-based irrigation zones to be chosen easily as input flow rate is not a concern. However, a variable speed pump does not help reduce problems with changing water availability, and such pumps are much more expensive to use and maintain than more commonly available pumps. Hence, there remains a need for an improved center pivot irrigation system that can handle varying input flow rates or water availability.
Another challenge to using center pivot irrigation systems is how to selectively water a field to leave some portions dry or non-irrigated. For example, a farmer may have a field that has one or more obstacles or areas where no crops are planted such as a rocky area or a road, and it may be a waste of precious and expensive water to irrigate all areas of the field uniformly as is presently the case with many center pivot irrigation system. One could turn off a nozzle or two, but obstacles generally are not circular or ring-shaped. This challenge also arises because certain portions of a field simply are not as productive as other areas (e.g., due to soil conditions or other reasons), and the farmer may wish to leave these areas of the field fallow for business reasons and only apply water to the best producing areas within a field (e.g., the farmer wants to be able to pick and choose where to irrigate). Often, the areas where irrigation is not as desirable will be very irregularly shaped and may be located in various, spaced-apart locations within the circular area covered by a center pivot irrigation system. Presently, there is no good solution on how to selectively irrigate a field with a center pivot irrigation system, and there remains a need for an improved control method or irrigation system that allows selective watering of areas or portions covered by a center pivot irrigation system while still maintaining uniform flow rates through the sprinkler arm (e.g., without using a variable speed pump(s)).